Electron beams can be formed with a very small diameter and have been used to write very high resolution patterns for microcircuit fabrication. Small diameter electron beams have been used not only as a direct writing lithography tool during fabrication of high density microcircuits but also indirectly to form high resolution masks for use in fabricating microcircuits.
Unfortunately, serial exposure of a high resolution electron beam pattern is very time consuming. The high cost of electron beam exposure systems and the relatively low thruput when high density lithographic exposure patterns of practical size are being formed (at best only a dozen pattern exposures per hour) has limited the direct manufacturing use of electron beam lithography, particularly as a direct writing microcircuit fabrication tool. There is a need for much higher thruput (i.e. more pattern exposures per hour per electron beam exposure system).
In an article by T. Barker and S. Bernacki entitled "Dual-Polarity, Single-Resist Mixed (E-Beam/Photo Lithography," IEEE Electron Device Letters, Vol. EDL-2, No. 11, pp. 281-283 (November 1981), a method is described in which small geometries or shapes in a pattern are exposed with an electron beam system and large geometries or shapes in the same pattern are exposed with light. It has been assumed that it will take less time to expose only the small shapes to an electron beam than it would take to expose all of the shapes in the pattern to an electron beam. Certainly this is true with some electron beam exposure systems. Unfortunately, upon development, the optically exposed shapes in the pattern will have a resolution and edge profile determined by the optical exposure system and the optical exposure/development process, while the electron beam exposed shapes in the pattern will have a resolution and edge profile determined by the electron beam system and the electron beam exposure/development process. This difference imposes additional process and pattern design constraints which are not present when the entire pattern is electron beam exposed. While a new or modified fabrication process or pattern design might be developed to take advantage of this method, it cannot be used as a practical matter as an exposure process which may be directly substituted in place of electron beam pattern exposure.
Another problem which has impeded more extensive use of high resolution electron beam lithography is "proximity effect." When an electron beam penetrates a material, such as an electron beam sensitive resist, the high energy primary electrons in the beam collide with molecules in the material and become laterally deflected or scattered in a random process. Secondary electrons are also released by the collisions and travel not only in the forward direction but also laterally and in the backward direction. Back scattering of secondary electrons is particularly great at material interfaces such as the boundary between an electron beam sensitive resist and a supporting substrate. The lateral spreading of the primary and secondary electrons causes nearby lateral regions to be exposed to scattered electrons. When two regions to be exposed are positioned close together, each receives not only the direct electron beam exposure during irradiation of that region, but also an extra dose of electron beam exposure due to the lateral scattering or spreading of electrons during exposure of the adjacent region. As a result, closely adjacent regions become more heavily exposed from the same incident dose than do isolated regions. Accordingly, when an electron beam sensitive resist is exposed to an entire pattern at a constant incident electron beam dose, electron beam exposed regions which are adjacent to other electron beam exposed regions develop differently than do isolated electron beam exposed regions. This has become known as the proximity effect.
A prior art method for avoiding proximity effects is to compensate for the expected added exposure contributed by electrons scattered from adjacent regions by correspondingly reducing the incident exposure dose of the region. Elaborate computer programs have been developed which calculate the dose needed at each elemental region of a complex pattern so as to get a substantially uniform total exposure everywhere in the pattern. Isolated regions get a greater incident exposure than regions adjacent to other regions.
While this technique is highly effective, it is also rather expensive due to the high cost of developing such programs and the extra expense in making the extensive computer calculations required, storing the dose data, and developing, fabricating and maintaining electron beam equipment having a computer controlled variable dose capability.
Another problem with the prior art technique of varying the exposure dose in order to compensate for proximity effect is that this technique cannot be used at all when the exposure process inherently produces a uniform incident exposure dose everywhere in the exposure pattern. This occurs, for example, if a mask is illuminated with an electron flood beam to form an electron beam exposure pattern or if a patterned layer directly emits a pattern of electrons. Such electron beam shadow masks and patterned electron emitters are not in practical use at this time but future development of such techniques is possible.
In a companion U.S. patent application, Ser. No. 431,241, filed Nov. 3, 1982 by Fletcher Jones for an "Electron Beam Lithography Proximity Correction Method," a technique is described for modifying an electron beam exposure pattern so as to make the exposure pattern less sensitive to proximity effects, without computing the exposure contribution from adjacent shapes due to lateral scattering and without requiring that the applied electron beam exposure dose be varied. Unfortunately, this technique does not significantly change the thruput of the electron beam exposure system.
It is an object of this invention to reduce the amount of electron beam exposure system time required in making a high resolution pattern exposure.
Another object is to improve electron beam exposure system thruput while maintaining electron beam resolution everywhere in the pattern and while maintaining a developed edge profile everwhere characteristic of an electron beam pattern exposure.
It is also an object of this invention to simultaneously reduce or eliminate electron beam proximity effects without requiring that the applied exposure dose be varied.
Still another object is to provide a pattern exposure method having improved electron beam exposure system thruput and reduced electron beam proximity effects without requiring computation of the exposure contribution from adjacent shapes due to lateral scattering effects.